refactor: replace better-sqlite3 with built-in node:sqlite

[sdirix]

Drops the only native dependency, so installs no longer risk a node-gyp build on platforms without prebuilt binaries (and the tree shrinks by 12 MB). Same SQLite file format, so existing reviews.db files keep working without migration.

• db.ts uses DatabaseSync, loaded lazily via createRequire so users on older Node get a readable error instead of ERR_UNKNOWN_BUILTIN_MODULE at startup; db.pragma(x) becomes db.exec('PRAGMA ' + x)
• add queryAll/queryOne helpers that keep the row-shape assertion in one place, since node:sqlite types every row as Record<string, SQLOutputValue> with no generic overloads; all row-returning queries in threads/tours/session go through them
• doctor checks node:sqlite availability instead of native module load
• engines.node >=18 -> >=22.13 (first release with node:sqlite unflagged)

As Node 18 and 20 are EOL already I think this is a good opportunity to slim down diffity. This makes life easier as diffity does not have to be rebuilt for every Node version. You might want to increase the major version before releasing to indicate breaking changes for users still using older Node versions.